The invention is based on a switch mode power supply. Such a switch mode power supply includes, inter alia, a periodically operated switching transistor and a transformer in order to achieve DC isolation and for producing operating voltages of different amplitude and polarity. In practice, efforts are being made to reduce the size of the transformer in order in this way to reduce costs and weight. Such a reduction in the size of the transformer is possible by increasing the operating frequency of the switch mode power supply, for example from 30 kHz to 120 kHz. However, in the case of such an operating frequency increase, the so-called switching-on losses of the switching transistor rise in proportion to the operating frequency and reach unacceptably high levels.
The object of the invention is to develop a switch mode power supply without any significant additional components such that the switching-on losses of the switching transistor are minimized and the transformer can be reduced in size without unacceptably increasing the switching-on losses.
In the case of the invention, the control transistor, which controls the switching transistor, is switched on during the isolating phase of the switching transistor until the collector voltage of the switching transistor has fallen to a minimum.
This results in the following advantageous effect. A switch mode power supply having an operating frequency of 25 kHz has switching-on losses on the switching transistor of approximately 0.6 W. If the operating frequency were increased by a factor 4 to approximately 100 kHz, the switching-on losses would thus rise to approximately 2.4 W. However, controlling the switching transistor according to the invention makes it possible to achieve switching-on losses of approximately 0.04 W. This advantageous effect is thus achieved merely by a particularly advantageous method of controlling the control transistor, while additional components are virtually unnecessary. Reducing the size of the transformer also allows the size of the printed circuit board (PCB=printed circuit board) which is fitted with the transformer to be reduced. In addition, as a result of the considerable reduction in the switching-on losses on the switching transistor, the heat sink which is provided for this transistor can also become smaller or can even be entirely omitted. The same applies to the resistor and the diode on the so-called snubber.
The control transistor is preferably controlled by an oscillator which carries out a plurality of functions. The oscillator produces the control voltage for the control transistor and, in consequence, for the switching transistor. The oscillator is additionally used as a starting oscillator, by one input of the oscillator being connected to the mains terminals. The oscillator is additionally used to vary the operating frequency of the switch mode power supply as a function of the respectively emitted power and, in consequence, enables a standby mode with a greatly reduced power of, for example, 5 W without so-called packet operation being required for controlling the switching transistor for this purpose. A voltage which is proportional to the mains power supply and is dependent on the power is applied to one control input of the oscillator and a voltage which is dependent on the amplitude of the control voltage, which is used for stabilization of the operating voltage produced, is applied to the other control input. In addition, a voltage is applied to the oscillator which voltage is derived from the control circuit and, in the standby mode, reduces the frequency of the oscillator and thus the operating frequency of the switch mode power supply as the power output drops. The oscillator thus makes it possible to control the switching transistor as a function of the mains voltage and of the power. At the same time, a charging path is located in the base path of the switching transistor, between the mains terminals and the isolating capacitor, which charging path is dimensioned such that the charging current of the isolating capacitor increases as the mains voltage rises.
The control transistor is preferably supplemented by a second transistor to form a circuit having a thyristor characteristic or a Darlington circuit.
In the case of a development, a partial winding of an additional primary winding of the transformer is connected to the isolating capacitor via a rectifier circuit. In this case, one end of the additional primary winding is connected via a rectifier circuit to a current measuring resistor at the emitter of the switching transistor.